The TestRetryExhaustion tests from PR #223 didn't mock time.sleep/time.time,
causing the retry backoff loops (275s+ total) to run in real time. Tests would
time out instead of running quickly.
Added _make_fast_time_mock() helper that creates a mock time module where
time.time() advances 500s per call (so sleep_end is always in the past) and
time.sleep() is a no-op. Both tests now complete in <1s.
The retry exhaustion checks used > instead of >= to compare
retry_count against max_retries. Since the while loop condition is
retry_count < max_retries, the check retry_count > max_retries can
never be true inside the loop. When retries are exhausted, the loop
exits and falls through to response.choices[0] on an invalid response,
crashing with IndexError instead of returning a proper error.
